Generic Lipitor is set to hit the U.S. market on Nov. 30. Analysts say that result will make it hard for the British drugmaker to argue patients would fare better on its Crestor than on much-cheaper generic Lipitor.

The study compared the drugs in high-risk patients with hardening of the arteries, which can cause strokes or heart attacks.

AstraZeneca released preliminary results Friday. Its U.S. shares fell more than 2 percent.